a. Field of the Invention
This invention relates to a design for loudspeakers, and more particularly relates to apex-driven woofer or cone-type loudspeakers with an improved acoustic transformer or phase plug.
b. Description of the Prior Art
In certain loudspeakers, particularly of the horn type, an acoustical transformer (commonly known as a "phase plug") is used to match the acoustic output from the speaker diaphragm to the surrounding listening area, either directly or through a horn, typically mounted in front of the phase plug/diaphragm combination. A phase plug is generally an insert fitted within the concavity defined by a loudspeaker and positioned in proximity to the speaker diaphragm. It is used to attempt to bring all portions of the sound waves generated by the diaphragm into phase coherence at the outlet (usually the throat of a horn) and to minimize the volume of the air chamber between itself and the diaphragm.
In the prior art, phase plugs have had various configurations. One commonly used phase plug has been the "concentric" or annular type, illustrated in Goldwater U.S. Pat. No. 4,157,741 and the paper of Bob H. Smith entitled "An Investigation of the Air Chamber of Horn Type Loudspeakers", at page 305 of the March 1953 issue (Vol. 3, No. 2) of the Journal of the Acoustical Society of America. A second prior art type has been the radial or "tangerine" type, shown in Henrickson U.S. Pat. No. 4,050,541 entitled "Acoustical Transformer for Horn-Type Loudspeakers". Usual phase plug design theory requires that the phase plug be structured with one or a series of evenly spaced "loading slots" disposed across the sound-radiating diaphragm, such as those defined by the concentric ring formation of the annular plug design or the pie-wedge-shaped formations of the radial plug design. To extend the high frequency performance of a loudspeaker (or compression driver), according to the standard concepts of phase plug theory, the phase plug should be closely spaced to the speaker diaphragm, the distance between the loading slots should be minimized, and the sound travel path lengths should be equal. These concepts have traditionally been thought to hold true for all speakers, and phase plug designs have been scaled accordingly to accommodate various sizes of speaker.
It was found that the traditional phase plug theory is not applicable over the entire frequency range of a woofer-type loudspeaker and in particular, an apex-driven woofer. For example, for a woofer speaker in one commonly used design, having a single, outwardly widening annular slot which is defined concentrically by an outer plug portion and an inner, centrally positioned "bullet" plug portion, instead of the theoretical frequency response with a peak in the output at 1.5 kHz and a "hole" (i.e., sharp dip) at 3 kHz, the actual measured response, indicated an unexplained peak at 3.2 kHz and two unexplained holes at 1.5 kHz and 5 kHz.
It is believed that this discrepancy is caused by the fact that standard phase plug theory assumes that the entire speaker diaphragm is vibrating as one piston. If the diaphragm was not vibrating as a piston (for example, if only the center portion of the diaphragm were vibrating) then concentric acoustic standing waves could exist between the faces of the diaphragm and the phase plug. The nulls in these standing waves could cause the appearance of previously unexpected holes or peaks.
Explained differently, when the entire diaphragm is vibrating, the boundary conditions are such that concentric standing waves in the air cavity or chamber between the diaphragm and the phase plug may be suppressed. If, however, there was a single driving point at the center of the speaker diaphragm with the outer portions of the diaphragm motionless, the boundary conditions would be proper for exciting concentric acoustic standing waves, producing the series of undesirable, well defined nulls and peaks in the speaker's frequency response. This condition, where only the center of the diaphragm vibrates, occurs in the relatively higher frequency ranges of woofer or apex-driven cone-type loudspeakers, to which the standard phase plug theory appears no longer to apply.